ABSTRACT The development and maturation of neurons require temporal induction of cell-specific gene expression programs. This is due in part to mechanisms of chromatin regulation that make sure the right genes are expressed and the wrong genes are repressed at any given time. While chromatin mechanisms that regulate neuronal differentiation early in development are well-characterized, the mechanisms in maturing fate-committed neurons are much less understood. One example of chromatin regulation is the trimethylation of lysine 27 on histone H3, which is a mark associated with gene repression and functions early on in development to prevent the expression of genes involved in alternative cell-fate determination. Our lab has shown that H3K27me3 is lost at specific sites during neuronal maturation, suggesting that this mark undergoes dynamic regulation over time. Thus, we raised the possibility that regulation of H3K27me3 by chromatin regulators can underlie neuronal maturation. Taking advantage of a well-defined model of neuronal development, the rodent cerebellum, I will characterize the role of Kdm6b, a lysine-specific demethylase involved in demethylating H3K27me3, in cerebellar maturation. Our lab has shown that loss of Kdm6b in cerebellar granule neurons (CGNs) results in a disruption of a mature gene expression program which includes many synaptic genes. Consistent with this, knockdown of Kdm6b in vivo in CGNs resulted in decreased density of PSD-95, a marker for the postsynapse. Thus, I hypothesize that Kdm6b regulates CGN synapse maturation through the temporal induction of this mature gene expression program. Utilizing a combination of powerful genetic tools and molecular biochemistry, I plan to study how loss of Kdm6b affects CGN synapse maturation in a cell-autonomous fashion in vivo over time, as well as dissect the mechanism by which Kdm6b regulates CGN gene expression. Completion of these two aims will define to what extent can changes in gene expression and chromatin regulation manifest as cellular changes in maturing fate-committed neurons. Additionally, this proposal will elucidate exactly how Kdm6b regulates genes to mediate downstream cellular changes.